Convert Inches of Mercury to Millimeters of Mercury

Formula

mmHg = inHg × 25.4

Derivation from SI units:

• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 mmHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.001 m = 133.322387415 Pa
• 1 inHg = 3386.388640341 Pa ÷ 133.322387415 Pa per mmHg = 25.4 mmHg

Both units are defined exactly, so the factor is exact. Inch of Mercury definition: inch of mercury; Millimeter of Mercury definition: millimeter of mercury.

Worked examples

1. A reading of 15 inHg converts to 381 mmHg.
   15 inHg × 25.4 = 381 mmHg
2. A value of 29 inHg is 736.6 mmHg.
   29 inHg × 25.4 = 736.6 mmHg

Where it's used

Inches of Mercury are used for aircraft altimeter settings (standard 29.92 inHg), while millimeters of mercury appear in blood pressure (e.g. 120/80 mmHg). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Inch of Mercury to other pressure units

Frequently asked questions

Sources

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "inch of mercury, conventional (inHg)".
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "millimeter of mercury, conventional (mmHg)".

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.